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TAsync is a javascript module to help the development of asynchronous code. It can be used both with node.js or directly in the browser. TAsync can be characterized as a future/promise library with some unique features:

  • follows stack traces across asynchronous calls
  • real and future values can be mixed freely
  • execution can be throttled in logical execution time

I use this library to work with a large database backend, where most objects are cached in memory. In this scenario you do not want to use regular callbacks, because either you call the callbacks before the methods return and then you run out of stack space, or you use nextTick which kills the performance. Also, traversing an extremely large tree asynchronously is hard: if you do it serially (depth first) then it is slow, if you do it parallel (breadth first) then you run out of memory, so you need a combination of the two. This (and much more) can be accomplished with the throttle functionality provided by this library.

Stack trace example

var TASYNC = require("tasync");
function divide (x, y) {
    if (=== 0) {
        throw new Error("divide by zero");
    return x / y;
function test() {
    var a = TASYNC.delay(100, 1);
    var b = TASYNC.delay(200, 0);
    return, a, b);
TASYNC.trycatch(test, function (error) {

In this example we crate two future values (a and b) that become available in 100 and 200 ms, then invoke the divide function which will throw an exception. The future result of the division is returned from the test function. When this value becomes available, we display the error trace on the console. Running this in node.js will display the following stack trace where line 15 is pointing to the return statement and line 7 to the throw statement.

Error: divide by zero
    at divide (~/tasync/test/teststack.js:7:9)
*** callback ***
    at test (~/tasync/test/teststack.js:15:16)
    at Object.trycatch (~/tasync/lib/tasync.js:435:16)
    at Object.<anonymous> (~/tasync/test/teststack.js:18:8)
    at Module._compile (module.js:441:26)
    at Object..js (module.js:459:10)
    at Module.load (module.js:348:32)
    at Function._load (module.js:308:12)
    at Array.0 (module.js:479:10)

Caching example

var fsReadFile = TASYNC.wrap(FS.readFile);
var lastFileName, lastFileData;
function cachedReadFile (fileName) {
    if (fileName === lastFileName) {
        return lastFileData + "\n";
    var futureFileData = fsReadFile(fileName);
    return, fileName, futureFileData);
function updateCache (fileName, fileData) {
    lastFileName = fileName;
    lastFileData = fileData;
    return fileData;
FS.readFile = TASYNC.unwrap(cachedReadFile);

In this example we monkey patch the node.js FS.readFile method to cache the last result and return that (with an extra end line character at the end) at subsequent calls with the same file name. We first turn a callback based method FS.readFile into a method that returns futures fsReadFile. Notice, that in cachedReadFile we either going to return a regular value or a future value. We can call fsReadFile directly, because we are sure that all parameters are regular values. However, the call to updateCache is done through since it has a parameter that is potentially a future object. The returns immediately, creating a new future that will be set when the updateCache call is eventually completed. Finally, we turn our future returning function cachedReadFile into a regular callback based one and monkey patch FS.readFile.

Throttle example

var fsReadDir = TASYNC.wrap(FS.readdir);
var fsStat = TASYNC.wrap(FS.lstat);
function readDir (dir) {
    var futureList = fsReadDir(dir);
    return, dir, futureList);
function processDir (dir, list) {
    for (var i = 0; i < list.length; ++i) {
        var filename = list[i];
        var filepath = dir + "/" + filename;
        var futureStat = fsStat(filepath);
        list[i] =, filename, filepath, futureStat);
    return TASYNC.apply(sum, list);
function processFile (filename, filepath, stat) {
    if (stat.isDirectory()) {
        return readDir(filepath);
    } else {
        return filename.indexOf(".js", filename.length - 3) !== -1 ? 1 : 0;
function sum () {
    var s = 0;
    for (var i = 0; i < arguments.length; ++i) {
        s += arguments[i];
    return s;

In this example the function readDir(dir) will search the given directory recursively and returns the number of javascript files found. The code is quite easy to follow (try to write the same with regular callbacks) and essentially performs a breadth first search. If you run this (or the equivalent code with callbacks) on a really really large directory, then you are going to run out of memory since you are creating potentially as many continuations as the largest breadth of your tree. If you replace the first line with this

var fsReadDir = TASYNC.throttle(TASYNC.wrap(FS.readdir), 5);

then you limit the number of concurrently executing FS.readdir calls to five. More importantly, when we select the next FS.readdir to be executed among the blocked ones, you select the directory whose name is the smallest in lexicographical order. This means, that you are approximating a serial (depth first) search but still perform up to five parallel calls. All of this is done by maintaining a tree of the outstanding futures where the lexicographical order is the logical time order of execution assuming your code is running sequentially.

Running the serial, parallel, original tasync, and throttled tasync versions of this program with hot caches we get the following execution times for the directory /usr/lib:

serial          996 ms
parallel        244 ms
tasync          348 ms
throttled       320 ms

With cold caches (sudo sh -c "echo 1 > /proc/sys/vm/drop_caches"), then the throttled version becomes the fastest:

serial          5680 ms
parallel        6533 ms
tasync          6540 ms
throttled       4975 ms


Most TAsync functions potentially return future objects. You should never call methods on these futures, nor should you test for them. Instead, use apply or call to invoke further functions when these potential future objects get resolved. Throwing of exceptions are encouraged and are properly handled throughout the library.

delay(timeout, value)

Returns a future value which will be resolved to value after timeout milliseconds. If timeout is negative, then value is returned immediately.

apply(func, args, [that])

Calls the func function with the args array of arguments on the optional that object. If one of the arguments is an unresolved future value, then this method returns a new future value that will be resolved when all arguments are resolved and the func function is returned. You can chain futures, that is, func can return a future value as well. If any of the arguments are futures that are rejected, then the returned future will be rejected with the same error. If all arguments are available (regular value, or a rejected or resolved future), then func will be called immediately and a regular value is returned or an exception is thrown.

call(func, arg1, ..., argn)

Same as apply(func, [arg1,...,argn], null).


Takes a node.js style asynchronous function func which should be called with a callback at the last argument, and turns it into a function that returns futures. In particular, if func calls the callback before returning, then the new function will return a regular object or throw an exception, otherwise it will return a future object which will be eventually resolved or rejected.


Takes a function that returns futures, and turns it into a node.js asynchronous function that takes a callback as the last parameter.

trycatch(func, handler)

Calls the func function with no parameters. If func throws an error or returns a future that is eventually rejected, then handler(error) is called. The result of the method will be the result of func if no error occurs, or the result of handler(error) if an error is detected. The error object passed to handle is an instance of Error and has an extra error.trace field that tracks the function calls across asynchronous calls.


Takes an array array of values and/or futures, and returns a future value that will be resolved to an array of values when all embedded futures are resolved. If one of the embedded futures is rejected, then the returned future will also be rejected.

throttle(func, limit)

Takes a function func that returns futures and turns it into another function which takes the same set of arguments, but ensues that no more than limit number of instances of func are concurrently running. If this limit is reached, then further calls of func are delayed until one of the running instances returns. This method chooses that outstanding call to run next which is earliest in the logical time ordering (i.e. the one that would be called fist if all asynchronous calls were synchronous).

join(first, second)

Returns first, when both first and second are resolved. If one of them are rejected, then that error is returned in the future. Both first and second can be regular objects, so this method may return a regular object, throw an error, or return a future.


Disables stack tracing across asynchronous calls. This makes the library slighly faster by omitting the creation of an extra object in each future.